The global spread of swine-origin influenza A (H1N1) viruses in humans in April 2009 marked the first influenza pandemic in 41 years. Over 35,000 people were infected with this novel H1N1 virus as of Jun. 15, 2009. Last century, an H1N1 influenza virus also caused the devastating 1918-19 pandemic. In addition, an H1N1 virus derived from swine caused an abortive pandemic in 1976. The 1918 influenza virus caused an influenza outbreak in the spring of 1918 and a lethal wave globally in the fall of that year, killing as many as 50 million people worldwide. Although the 2009 swine-origin H1N1 influenza virus was viewed as mild in early 2009, the possibility exists that this virus may mutate and become more virulent in the future. Thus, there is a need to develop effective vaccines to prevent future outbreaks caused by H1N1 viruses.